Human - chimp (min 6.5 Ma; max 10 Ma)

The dating of the chimp-human split has been discussed for nearly a century. Early paleontological estimates, up to the 1970s, placed the branching point deep in the Miocene, at perhaps 20 - 15 Ma, but this was revised dramatically upwards to about 5 Ma by early molecular studies (Sarich and Wilson 1967), and estimates as low as 2.7 Ma have been quoted (Hasegawa, Kishino, and Yano 1985). Paleontological evidence for the branching point was distinctly one-sided until recently, since the only fossils fell on the human line, and so the question of the date of divergence of humans and chimps became synonymous, for paleontologists, with the date of the oldest certain hominin (species on the human, not chimp, line). The recent discovery of the first chimpanzee fossils (McBrearty and Jablonski 2005) does not change much, as they are dated as 545,000 years old at most.

The date of the oldest hominin has extended backwards rapidly in the last 25 years. Until 1980, the oldest fossils were gracile and robust australopithecines from 3 Ma. The discovery of ‘Lucy’, now termed Praeanthropus afarensis in Ethiopia (Johanson and Taieb 1976) extended the age back to 3.2 Ma at most. Then, two further hominin species pushed the age back to over 4 Myr: Ardipithecus ramidus from rocks dated as 4.4 Ma from Ethiopia (White et al. 1994) and Praeanthropus anamensis from rocks dated as 4.1 - 3.9 Ma from Kenya (Leakey et al. 1995). More recent finds, remarkably, have pushed the dates back to 6 Myr: Ardipithecus ramidus kadabba from Ethiopia (5.8 - 5.2 Ma; Haile-Selassie 2001), Ororrin tugenenis from Kenya (c. 6 Ma; Senut et al. 2001), and Sahelanthropus tchadensis from Chad (6-7 Ma; Brunet et al. 2002). The last two taxa have proved highly controversial, with claims that one or other, or both, are not hominin, but ape-like. However, the majority view is that Sahelanthropus at least is hominin (Wood 2002; Cela-Conde & Ayala 2003), and so its date becomes crucial.

Dating of the Sahelanthropus beds in Chad is not direct. Biostratigraphic evidence from mammals in particular, but with cross-checking from fish and reptile specimens, indicates that the unit is definitely late Miocene (i.e. older than 5.33 Ma), and it is older than the Lukeino Formation of Kenya, the source of Orrorin (dated at 6.56 - 5.73 Ma from Ar/Ar dates on volcanic layers; Deino et al. 2002), and may be equivalent to the lower fossiliferous units of the Nawata Formation at Lothagam (dated as 7.4 - 6.5 Ma; Vignaud et al. 2002). This might suggest a date for the sediments containing Sahelanthropus of 7.5 - 6.5 Ma, based on biostratigraphy and external dating. Thus, we determine a 6.5 Ma age for the minimum constraint on the human-chimp split. Kumar et al. (2005) have recently calculated a range of ages for the human-chimp divergence of 4.98-7.02 Ma; their minimum constraint (4.98 Ma) is younger than the oldest fossils (Orrorin, Sahelanthropus). However, paleoanthropologists generally accept that Sahelanthropus and Orrorin were both bipedal, upright forms, and until both are rejected by consensus view of their anatomy, we retain them as the oldest valid hominins.

A soft maximum constraint on the human-chimp divergence is hard to place because the immediate outgroups (gorilla, orang, gibbons) lack convincing fossil records. Some late Miocene ape fossils, such as Gigantopithecus and Sivapithecus may be stem-orangs. Nonetheless, a range of such apes, Ankarapithecus from Turkey (10 Ma), Gigantopithecus from China (8 – 0.3 Ma), Lufengopithecus from China (10 Ma), Ouranopithecus from Greece (10 – 9 Ma), and Sivapithecus from Pakistan (10 – 7 Ma) give maximum ages of 10 Ma, early in late Miocene, and these deposits have yielded no fossils attributable to either chimps or humans. This is taken as the soft maximum constraint on the human-chimp divergence.

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